SQL 查询 - 如何在 WHERE 子句中放置条件

Question

我是 SQL 的新手，我想知道是否可以在 WHERE 子句之后添加一个条件。更具体地说，我试图在我的数据库中找出 "which item have been sold to two people"。

我已经成功地连接了所有卖给人们的物品，并且想要一个只给出卖给两个人的物品的条件。我试过在 WHERE 子句中使用 COUNT(customer_id) 但它给出了 "aggregates not allowed in WHERE clause"。

编辑：

数据库 表是：

books((book_id), title, author_id, subject_id) 
publishers((publisher_id), name, address) 
authors((author_id), last_name, first_name) 
stock((isbn), cost, retail_price, stock) 
shipments((shipment_id), customer_id, isbn, ship_date) 
customers((customer_id), last_name, first_name) 
editions((isbn), book_id, edition, publisher_id, publication_date) 
subjects((subject_id), subject, location)

我当前的查询是：

SELECT title, first_name, last_name 
FROM books, shipments, customers, editions 
WHERE books.book_id = editions.book_id 
AND editions.isbn = shipments.isbn 
AND shipments.customer_id = customers.customer_id 
AND COUNT(customers.customer_id) = 2;

谢谢大家！

Answer 1

您需要使用 HAVING 子句，如下所示：

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator val

继续；

Answer 2

您应该学习如何使用正确的 join 语法和 table 别名：

SELECT b.title, c.first_name, c.last_name 
FROM books b join
     editions e
     on b.book_id = e.book_id  join
     shipments s
     on e.isbn = s.isbn join
     customers c
     on s.customer_id = c.customer_id
GROUP BY title, first_name, last_name 
HAVING COUNT(customers.customer_id) = 2;

您问题的答案是 HAVING 子句和 GROUP BY.

SQL 查询 - 如何在 WHERE 子句中放置条件

SQL query - how to put conditions in WHERE clause

sql

postgresql

where